U.S. patent number 11,171,013 [Application Number 16/390,223] was granted by the patent office on 2021-11-09 for leveraging precursor molecular composition and structure for atomic layer etching.
This patent grant is currently assigned to University of Maryland, College Park. The grantee listed for this patent is University of Maryland. Invention is credited to Chen Li, Kang-Yi Lin, Gottlieb S. Oehrlein.
United States Patent |
11,171,013 |
Oehrlein , et al. |
November 9, 2021 |
Leveraging precursor molecular composition and structure for atomic
layer etching
Abstract
Provided is a method of selectively etching a substrate
comprising at least one cycle of: depositing a chemical precursor
on a surface of the substrate to form a chemical precursor layer on
the substrate, the substrate comprising a first portion and a
second portion, wherein the first and the second portion are of a
different composition; selectively removing the chemical precursor
layer and at least a part of the first portion of the substrate;
and repeating the cycle until the first portion of the substrate is
substantially or completely removed, wherein deposition of the
chemical precursor and selective removal of the chemical precursor
layer and at least a part of the first portion of the substrate are
performed under a plasma environment.
Inventors: |
Oehrlein; Gottlieb S.
(Clarksville, MD), Lin; Kang-Yi (Greenbelt, MD), Li;
Chen (College Park, MD) |
Applicant: |
Name |
City |
State |
Country |
Type |
University of Maryland |
College Park |
MD |
US |
|
|
Assignee: |
University of Maryland, College
Park (College Park, MD)
|
Family
ID: |
1000005920172 |
Appl.
No.: |
16/390,223 |
Filed: |
April 22, 2019 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20200335352 A1 |
Oct 22, 2020 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L
21/32136 (20130101); H01L 21/31116 (20130101); H01L
21/32137 (20130101); H01L 21/02019 (20130101); H01L
21/3065 (20130101); H01L 21/32135 (20130101); H01L
21/02205 (20130101); H01L 21/0228 (20130101); C23C
16/45553 (20130101); H01L 21/02274 (20130101) |
Current International
Class: |
H01L
21/3213 (20060101); H01L 21/311 (20060101); H01L
21/02 (20060101); C23C 16/455 (20060101); H01L
21/3065 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
C Li et al., "Fluorocarbon based atomic layer etching of Si3N4 and
etching selectivity of SiO2 over Si3N4", J. Vac. Sci. Technol. A,
vol. 34, pp. 041307-1 to 041307-12. (Year: 2016). cited by examiner
.
Engelmann et al., "Plasma-surface interactions of model polymers
for advanced photoresists using C.sub.4F.sub.8/Ar discharges and
energetic ion beams," Journal of Vacuum Science & Technology B:
Microelectronics and Nanometer Structures Processing, Measurement
and Phenomena 25, 1353 (2007). cited by applicant .
Lin et al., "Achieving ultrahigh etching selectivity of SiO2 over
Si3N4 and Si in atomic layer etching by exploiting chemistry of
complex hydrofluorocarbon precursors," Journal of Vacuum Science
& Technology A 36, 040601 (2018). cited by applicant .
Li et al., "Fluorocarbon based atomic layer etching of
Si.sub.3N.sub.4 and etching selectivity of SiO.sub.2 over
Si.sub.3N.sub.4," Journal of Vacuum Science & Technology A 34,
041307 (2016). cited by applicant .
Li et al., "Impact of hydrofluorocarbon molecular structure
parameters on plasma etching of ultra-low-K dielectric," Journal of
Vacuum Science & Technology A 34, 031306 (2016). cited by
applicant .
Metzler et al. "Fluorocarbon assisted atomic layer etching of
SiO.sub.2 and Si using cyclic Ar/C.sub.4F.sub.8 and Ar/CHF.sub.3
plasma," Journal of Vacuum Science & Technology A 34, 01B101
(2016). cited by applicant .
Metzler et al., "Fluorocarbon assisted atomic layer etching of
SiO.sub.2 using cyclic Ar/C.sub.4F.sub.8 plasma," Journal of Vacuum
Science & Technology A 32, 020603 (2014). cited by applicant
.
Standaert et al., "Patterning of fluorine-, hydrogen-, and
carbon-containing SiO.sub.2-like low dielectric constant materials
in high-density fluorocarbon plasmas: Comparison with SiO.sub.2,"
Journal of Vacuum Science & Technology A 17, 741 (1999). cited
by applicant.
|
Primary Examiner: Lu; Jiong-Ping
Attorney, Agent or Firm: Stanek Lemon Crouse & Meeks,
PA
Claims
That which is claimed:
1. A method of selectively etching a substrate comprising at least
one cycle of: depositing a chemical precursor on a surface of the
substrate to form a chemical precursor layer on the substrate, the
substrate comprising a first portion and a second portion, wherein
the first portion is of a different composition than that of the
second portion; selectively removing the chemical precursor layer
and at least a part of the first portion of the substrate; and
repeating the cycle until the first portion of the substrate is
substantially or completely removed, with minimal or no
removal/loss of the second portion of the substrate, wherein
deposition of the chemical precursor and selective removal of the
chemical precursor layer and at least a part of the first portion
of the substrate are performed under a plasma environment, wherein
the first portion of the substrate comprises SiO.sub.2, wherein the
second portion of the substrate comprises Si.sub.3N.sub.4, and
wherein the chemical precursor is a hydrofluorocarbon (HFC) having
of chemical formula C.sub.kH.sub.mF.sub.n, wherein k is 2, 3, or 4,
m is an integer from 1 to 2k+1 inclusive, n is an integer from 1 to
2k+1 inclusive, and m=n.
2. The method of claim 1, wherein the first portion of the
substrate is on the second portion of the substrate.
3. The method of claim 1, wherein the chemical precursor layer has
a thickness of at least about 7 .ANG. to about 3 nm.
4. The method of claim 1, wherein less than about 4 .ANG. of the
second portion is removed.
5. The method of claim 1, wherein the hydrofluorocarbon is selected
from the group consisting of (E)-1,1,1,3-tetrafluoro-2-butene,
(Z)-1,1,1,3-tetrafluoro-2-butene, 2,4,4,4-tetrafluoro-1-butene,
2,3,3-trifluoropropene, 3,3,3-trifluoropropene,
1,1,3-trifluoroprop-1-ene, 1,2,3-trifluoroprop-1-ene,
1,3,3-trifluoroprop-1-ene, 1,1,1-trifluoroethane, and
1,1,2-trifluoroethane.
6. The method of claim 1, wherein the hydrofluorocarbon is
3,3,3-trifluoropropene (F.sub.3C--CH.dbd.CH.sub.2).
7. A method of atomic layer etching comprising at least one cycle
of: depositing a chemical precursor comprising fluorine on a
surface of a substrate to form a fluorocarbon film on the surface
of a first portion of the substrate, the substrate comprising the
first portion on a second portion of the substrate, wherein the
first portion is of a different composition than that of the second
portion; selectively removing the fluorocarbon film and at least a
part of the first portion of the substrate; and repeating the cycle
until the first portion of the substrate is substantially or
completely removed, with minimal or no removal/loss of the second
portion of the substrate, wherein deposition of the chemical
precursor and selective removal of the fluorocarbon film and at
least a part of the first portion of the substrate are performed
under a plasma environment, wherein the first portion of the
substrate comprises SiO.sub.2, wherein the second portion of the
substrate comprises Si.sub.3N.sub.4, and wherein the chemical
precursor is a hydrofluorocarbon (HFC) having of chemical formula
C.sub.kH.sub.mF.sub.n, wherein k is 2, 3, or 4, and m is an integer
from 1 to 2k+1 inclusive, n is an integer from 1 to 2k+1 inclusive,
and m=n.
8. The method of claim 7, wherein the hydrofluorocarbon is selected
from the group consisting of (E)-1,1,1,3-tetrafluoro-2-butene,
(Z)-1,1,1,3-tetrafluoro-2-butene, 2,4,4,4-tetrafluoro-1-butene,
2,3,3-trifluoropropene, 3,3,3-trifluoropropene,
1,1,3-trifluoroprop-1-ene, 1,2,3-trifluoroprop-1-ene,
1,3,3-trifluoroprop-1-ene, 1,1,1-trifluoroethane, and
1,1,2-trifluoroethane.
9. The method of claim 7, wherein the fluorocarbon film has a
thickness of at least about 7 .ANG. to about 3 nm.
10. The method of claim 7, wherein less than about 4 .ANG. of the
second portion is removed.
11. The method of claim 7, wherein the hydrofluorocarbon is
3,3,3-trifluoropropene (F.sub.3C--CH.dbd.CH.sub.2).
12. A method of selectively etching a substrate comprising at least
one cycle of: depositing a hydrofluorocarbon having a chemical
formula C.sub.kH.sub.mF.sub.n on an SiO.sub.2 first material layer
of the substrate to form a hydrofluorocarbon film on a surface of
the SiO.sub.2 first material layer of the substrate, the substrate
comprising the SiO.sub.2 first material layer on an Si.sub.3N.sub.4
second material layer; selectively removing the hydrofluorocarbon
film and at least a part of the SiO.sub.2 first material layer; and
repeating the cycle until the SiO.sub.2 first material layer of the
substrate is substantially or completely removed, with minimal or
no removal/loss of the Si.sub.3N.sub.4 second material layer,
wherein k of chemical formula C.sub.kH.sub.mF.sub.n is 2, 3, or 4,
m is an integer from 1 to 2k+1 inclusive, n is an integer from 1 to
2k+1 inclusive and m=n, wherein deposition of the hydrofluorocarbon
and selective removal of the SiO.sub.2 first material layer of the
substrate are performed under a plasma environment comprising Argon
(Ar), and wherein less than about 4 .ANG. of the Si.sub.3N.sub.4
second material layer is removed.
13. The method of claim 12, wherein the hydrofluorocarbon is
selected from the group consisting of
(E)-1,1,1,3-tetrafluoro-2-butene, (Z)-1,1,1,3-tetrafluoro-2-butene,
2,4,4,4-tetrafluoro-1-butene, 2,3,3-trifluoropropene,
3,3,3-trifluoropropene, 1,1,3-trifluoroprop-1-ene,
1,2,3-trifluoroprop-1-ene, 1,3,3-trifluoroprop-1-ene,
1,1,1-trifluoroethane, and 1,1,2-trifluoroethane.
14. The method of claim 12, wherein the hydrofluorocarbon is
3,3,3-trifluoropropene (F.sub.3C--CH.dbd.CH.sub.2).
15. The method of claim 12, wherein the hydrofluorocarbon film has
a thickness of at least about 7 .ANG. to about 3 nm.
16. The method of claim 12, wherein less than about 4 .ANG. of the
second material layer is removed.
Description
BACKGROUND
The development of atomic layer etching (ALE) approaches has many
promising performance characteristics, including simple process
steps, self-limited etching, improved uniformity, and potentially
ultrahigh material etch selectivity required for advanced
semiconductor manufacturing. Utilization of the chemical nature of
precursors may have significant potential for advancements in ALE
over conventional steady-state plasma etching, enabling greater
control over surface reactions. Manipulating the chemical nature of
precursors in an ALE process may lead to novel processing margins
for Angstrom-level etch controllability which to date have not been
attainable.
The interaction between plasma dissociated reactive gas and
substrate significantly affects etching results, including etching
selectivity and pattern transfer fidelity. In continuous wave (CW)
plasma, the effect of leveraging the chemical structure of the
precursor has comparable results as the effect of the precursor
mixture. It has been shown that the correlation between the
CF.sub.2 fraction in the precursor gas and in the deposited film
surface chemistry is less than the correlation between the CF.sub.2
fraction in the precursor gas and in the plasma gas phase obtained
from optical emission spectroscopy. It has also been suggested that
the effect of leveraging the content or molecular structure in
hydrofluorocarbon (HFC) precursor is similar in effect to that of
fluorocarbon (FC) precursor gas mixing with hydrogen additives.
Recently, FC-assisted ALE using C.sub.4F.sub.8 and CHF.sub.3 on
SiO.sub.2 and Si etching, and using C.sub.4F.sub.8 and CHF.sub.3 on
Si.sub.3N.sub.4 etching, indicate that the chemical structure of a
precursor has an influence on the fluorine and carbon content
deposited on the surface. The CHF.sub.3-based ALE resulted in a
higher fluorination level on the Si.sub.3N.sub.4 surface than
C.sub.4F.sub.8, which is different from the observations in CW
plasma. CH.sub.3F-based ALE of Si.sub.3N.sub.4 has been
investigated and exhibit ALE selectivity of Si.sub.3N.sub.4 over
Si. However, the potential for further improving the performance of
ALE, such as improving etch selectivity, through modifications in
chemical composition and/or structure of the chemical precursor,
has not been established and is still not well characterized or
understood.
SUMMARY
According to an aspect of the invention, provided is a method of
selectively etching a substrate including at least one cycle of:
depositing a chemical precursor on a surface of the substrate to
form a chemical precursor layer on the substrate, the substrate
including a first portion and a second portion, wherein the first
and the second portion are of a different composition; selectively
removing the chemical precursor layer and at least a part of the
first portion of the substrate; and repeating the cycle until the
first portion of the substrate is substantially or completely
removed, wherein deposition of the chemical precursor and selective
removal of the chemical precursor layer and at least a part of the
first portion of the substrate are performed under a plasma
environment.
According to another aspect of the invention, provided is a method
of atomic layer etching including at least one cycle of: depositing
a chemical precursor including fluorine on a surface of a first
portion of a substrate to form a fluorocarbon film on the surface
of the first portion of the substrate, the substrate including the
first portion on a second portion of the substrate, wherein the
first and the second portion are of a different composition;
selectively removing the fluorocarbon film and at least a portion
of the first portion of the substrate; and repeating the cycle
until the first portion of the substrate is substantially or
completely removed, wherein deposition of the chemical precursor
and selective removal of the fluorocarbon film and at least a part
of the first portion of the substrate are performed under a plasma
environment.
According to another aspect of the invention, provided is a method
of selectively etching a substrate including at least one cycle of:
depositing a hydrofluorocarbon on an SiO.sub.2 first portion of the
substrate to form a hydrofluorocarbon film on the surface of the
substrate, the substrate including the SiO.sub.2 first portion on
an c-Si, Si, Si.sub.3N.sub.4 or SiGe second material layer;
selectively removing the hydrofluorocarbon film and at least a part
of the SiO.sub.2 first portion; and repeating the cycle until the
SiO.sub.2 first material, layer of the substrate is substantially
or completely removed, wherein deposition of the hydrofluorocarbon
and selective removal of the SiO.sub.2 first portion of the
substrate are performed under a plasma environment including Argon
(Ar).
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a) and 1(b). FIG. 1(a) illustrates ellipsometry modeling
for SiO.sub.2--Si.sub.3N.sub.4--SiO.sub.2 stack sample with top
SiO.sub.2 etching (solid arrow, bottom SiO.sub.2 layer is not
shown) and underlayer Si.sub.3N.sub.4 etching (short dotted line).
The dashed line describes the formation of a FC passivation layer
with 4 and 7 nm Si.sub.3N.sub.4 loss before etch stop. FIG. 1(b)
illustrates the experimental in situ ellipsometry data of ALE with
C.sub.4F.sub.8, C.sub.4F.sub.8/H.sub.2, and C.sub.3H.sub.3F.sub.3
superimposed on the ellipsometry model. The figure shows various
Si.sub.3N.sub.4 losses by using different precursors and
essentially negligible Si.sub.3N.sub.4 loss in the case of
C.sub.3H.sub.3F.sub.3.
FIGS. 2(a) and 2(b). FIG. 2(a) illustrates SiO.sub.2 and
Si.sub.3N.sub.4 layer thickness at the end of each ALE cycle using
C.sub.4F.sub.8, C.sub.4F.sub.8/H.sub.2 and C.sub.3H.sub.3F.sub.3,
respectively. Si.sub.3N.sub.4 etching stops at various thicknesses.
FIG. 2(b) illustrates the amount of Si.sub.3N.sub.4 thickness loss
before etching stops for three conditions.
FIGS. 3(a)-3(l) illustrate Si.sub.3N.sub.4 XPS spectra obtained at
a take-off angle of 20.degree. that compare surfaces after ALE
using: FIGS. 3(a)-3(c) C.sub.4F.sub.8; FIGS. 3(d)-3(f)
C.sub.4F.sub.8/H.sub.2; and FIGS. 3(g)-3(i) C.sub.3H.sub.3F.sub.3
precursors at the end of deposition and etching steps,
respectively. XPS difference spectra formed by subtracting XPS data
after the etching step from the data after the deposition step are
also shown for the three chemistries (C.sub.4F.sub.8,
C.sub.4F.sub.8/H.sub.2, and C.sub.3H.sub.3F.sub.3). The spectra
include; FIG. 3(j) Si.sub.2p; FIG. 3(k) C1s; and FIG. 3(l) F1s.
FIGS. 4(a) and 4(b). FIG. 4(a) illustrates a comparison of the F/C
ratios from the C1s and F1s/C1s spectra with ALE using
C.sub.4F.sub.8, C.sub.4/H.sub.2 and C.sub.3H.sub.3F.sub.3 at the
end of deposition and etching steps, respectively. FIG. 4(b)
illustrates the thickness of the HFC film remained on the
Si.sub.3N.sub.4 surface at the end of etching step for the three
precursors.
FIGS. 5(a) and 5(b). FIG. 5(a) illustrates Si.sub.2 and Si
thicknesses at the end of each ALE cycle using C.sub.4F.sub.8,
C.sub.4F.sub.8/H.sub.2 and C.sub.3H.sub.3F.sub.3 precursors. FIG.
5(b) illustrates precursor chemistry dependence of Si thickness
loss prior to Si etch stop as the result of formation of a
passivation layer.
DETAILED DESCRIPTION
The foregoing and other aspects of the present invention will now
be described in more detail with respect to other embodiments
described herein. It should be appreciated that the invention can
be embodied in different forms and should not be construed as
limited to the embodiments set forth herein. Rather, these
embodiments are provided so that this disclosure will be thorough
and complete, and will fully convey the scope of the invention to
those skilled in the art.
The terminology used in the description of the invention herein is
for the purpose of describing particular embodiments only and is
not intended to be limiting of the invention. As used in the
description of the invention and the appended claims, the singular
forms "a", "an" and "the" are intended to include the plural forms
as well, unless the context clearly indicates otherwise.
Additionally, as used herein, the term "and/or" includes any and
all combinations of one or more of the associated listed items and
may be abbreviated as "/".
Unless otherwise defined, all technical and scientific terms used
herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs.
According to embodiments of the invention, highly selective atomic
layer etching (ALE) of semiconductor materials can be provided
using, in some embodiments, hydroflurocarbon (HFC) precursors.
The term atomic layer etching, as used herein, can broadly refer to
the level of layer dimensional control that can be achieved at the
angstrom (.ANG.) level. Thus, atomic layer etching may generally
correspond to the size of atoms. Additionally, the average removed
layer thickness/cycle can be less than 1 .ANG. per etching cycle,
or more than 1 .ANG. per cycle, for example, about 3 .ANG., about 2
.ANG., about 3 .ANG., about 4 .ANG., about 5 .ANG., about 6 .ANG.,
about 7 .ANG., about 8 .ANG., about 9 .ANG., about 10 .ANG. nm),
about 11 .ANG., about 12 .ANG., about 13 .ANG., about 14 .ANG.,
about 15 .ANG., about 16 .ANG., about 17 .ANG., about 18 .ANG.,
about 19 .ANG., about 20 .ANG. (2 nm), about 2.5 nm, or about 3 nm
per cycle.
Although the following discussion mainly focuses on the
hydrofluorocarbon or fluorocarbon, the concept can be applied to
other precursors, including complex compounds that contain
chlorine, boron trichloride, bromine, oxygen, ammonia and others
employed in a plasma etching process. The methods of generating
plasma are not limited to the inductively coupling plasma (ICP),
for example conductive coupling plasma (CCP), helical plasma or
electron cycle resonance (ECR) plasma may also be adopted to
methods of the present invention. In some embodiments, the
generated plasma and the plasma environment used in the plasma
etching process according to methods of the present invention is a
plasma including Argon (Ar), for example, such as a steady-state Ar
plasma generated in an inductively coupled plasma (ICP)
reactor.
The term substrate, as used herein, can broadly refer to any layer
upon which processing is desired. Thus, for example, a native oxide
film on the surface of a silicon substrate may itself be considered
a substrate for the purposes of this discussion. Likewise, layers
deposited on silicon or on other base substrates may likewise be
considered substrates in some embodiments. For example, in some
embodiments, a multi-layer stack may be formed, and then atomic
layer etching may be performed on the top layer of the stack. In
such a case, the top layer may be considered the substrate. In
general, the layer or layers upon which the chemical precursor is
deposited and/or which reacts with the chemical precursor can be
considered the substrate layer(s).
Various substrates may be used in studying and evaluating the use
of cyclic plasma interacting with the substrate. For example, such
substrates may include materials that show chemically enhanced
etching in the presence of low energy bombardment and a chemical
precursor. The substrates can also include at least one of
SiO.sub.2, Si.sub.3N.sub.4, Silicon (Si) amorphous Si, crystalline
Si (c-Si), monocrystalline. Si, polycrystalline Si,
Si.sub.xGe.sub.1-x, GaAs or other group III-V semiconductors,
GaAl.sub.xAs.sub.1-x, InGaAs, GaP.sub.xAs.sub.1-x, or the oxides,
nitrides, or oxynitrides of any of the above listed substrate
materials, or like materials other than those listed above.
Additionally, the substrate may have a native oxide layer on the
surface of the substrate. Moreover, a thickness of the native oxide
layer may vary. For example, the thickness of the native oxide
layer may be about 1/10 of 1 nm (1 .ANG.) to about 10 nm. In some
embodiments, the substrate can include SiO.sub.2, and at least one
of Si, c-Si, Si.sub.3N.sub.4 and/or SiGe. Ln other embodiments, the
substrate can include SiO.sub.2, and at least one of Si and/or
Si.sub.3N.sub.4. In some embodiments, the substrate can include
SiO.sub.2, and Si.sub.3N.sub.4. In other embodiments, the substrate
can include SiO.sub.2, and Si. In other embodiments, the substrate
may include an SiO.sub.2 layer on a layer including: at least one
of Si, c-Si, Si.sub.3N.sub.4 and/or SiGe; Si and/or
Si.sub.3N.sub.4; Si.sub.3N.sub.4; and Si. In some embodiments, the
substrate may include an SiO.sub.2 layer on a Si.sub.3N.sub.4
layer. In other embodiments, the substrate may include an SiO.sub.2
layer on an Si layer.
The substrate, according to other embodiments, may include, but are
not limited to, wafers. The wafers may be of various suitable
processing sizes. For example, substrate samples may be on the
order of 1 inch or smaller, if desired. Additionally, full wafers
of about 200 mm or about 300 mm in diameter, or even larger
substrates may be incorporated.
According to some embodiments, angstrom or angstroms (.ANG.) thick
precursor layers can be provided/deposited on a surface layer of a
substrate. For example, HFC, or fluorocarbon (FC), chemical
precursor layers can be deposited to a thickness of about 1 .ANG.,
2 .ANG., 3 .ANG., 4 .ANG., 5 .ANG., 6 .ANG., 7 .ANG., 8 .ANG., 9
.ANG., 10 .ANG. (1 nm), 1.5 nm, 2 nm, 2.5 nm, to about 3 nm on a
surface layer to form a modified surface layer, for example, such
as a modified SiO.sub.2 surface layer. In some embodiments, the
thickness of the chemical precursor, layers should be of sufficient
thickness to provide an etch selectivity between substrate
materials, for example SiO.sub.2 to Si.sub.3N.sub.4 or SiO.sub.2 to
Si. In some embodiments, the thickness of the precursor layer is at
least about 7 .ANG., about 10 .ANG., about 15 .ANG., about 20 .ANG.
to about 3 nm. The stoichiometry of the HFC or FC precursor layers
is variable, and they may include other elements not specifically
called out, such as, for example, H. Subsequently, after
establishing a gaseous environment of sufficient purity, such as,
for example, by maintaining an Ar environment without additional
precursor injection, low energy Ar.sup.+ ion bombardment can be
applied to remove portions of the precursor layer, portions of the
reacted surface layer, and controlled portions of the surface
layer. This may be followed by another period during which an Ar
gaseous environment of sufficient purity is established. Such a
process can be repeated in a cyclical manner until a desired
overall etching depth has been achieved. Further, low energy ion
bombardment can be applied in various eV ranges, such as, for
example, within a range of about 0 eV up to about 100 eV. In other
embodiments, gases such as Ar, including Ne and/or Xe may also be
applied.
The term "fluorocarbon" (FC) may refer to an organofluorine
compound containing only fluorine and carbon, such as a
perfluorocarbon, perfluoroalkane, perfluoroalkene, and the like,
however, is not strictly limited thereto, and may refer to, in some
embodiments, a hydrocarbon in which some or all C--H bonds have
been replaced by C--F bonds. An example of a hydrocarbon in which
some C--H bonds have been replaced by C--F bonds, is a
hydrofluorocarbon (HFC), such as CHF.sub.3, C.sub.3H.sub.3F.sub.3,
or any C.sub.kH.sub.mF.sub.n, or isomers thereof.
An ALE cycle may include a deposition step in which a precursor is
injected into a reactor containing the substrate, such as an ion
coupled plasma (ICP) reactor, for a period of time, for example, 1
s, for depositing thinner precursor film layers, such as a 5 .ANG.
precursor film layer, or 2 s, for depositing thicker precursor film
layers, such as a 10 .ANG. precursor film layer, on the substrate,
followed by a purge step to pump out residual reactive gas. In some
embodiments, the purge step is about 12 s. The purge step is
followed by an etching step performed by applying, for example, an
RF bias voltage for a predetermined length of time, as will be
further described below.
Material etching can stop once the reacted surface layer and/or a
portion of the surface layer has been removed, resulting in a
self-limited process. Thus, according to some embodiments,
development of atomic layer etching processes for complex materials
may be feasible.
In some embodiments, various precursors may be applied in ALE. For
example, applicable precursors may include oxygen gas and
fluorocarbon gas, such as, for example, FC precursors such as
C.sub.4F.sub.8, C.sub.4F.sub.6, or CF.sub.4. In some embodiments.
HFC precursors, such as CHF.sub.3, C.sub.3F.sub.3H.sub.3, any
C.sub.kH.sub.mF.sub.n, or isomers thereof, may be applied. In some
embodiments, k, m, and n of chemical formula C.sub.kH.sub.mF.sub.n
are each independently, an integer between, and including, 1 and
about 6 for k, and an integer between, and including, 1 and about
2k+1 for m and n, respectively. In some embodiments, k is an
integer between, and including, 2 and 4. In some embodiments, n is
an integer between, and including, k and 2k+1. In some embodiments,
n=m. Additionally, in some embodiments, Cl.sub.2 or at least one
Br-based gas alone, or in combination with fluorocarbon gases may
also be applied. Moreover, in other embodiments, the precursor may
include at least one of CHF.sub.3, C.sub.3F.sub.3H.sub.3,
C.sub.kH.sub.mF.sub.n gas precursors or isomers thereof, or any
C.sub.pO.sub.qF.sub.r gas precursors or isomers thereof, either
alone or with admixtures of N.sub.2, H.sub.2, O.sub.2, CO,
CO.sub.2, noble gases, or SiF.sub.4, either alone or in any
combination thereof. In some embodiments, the precursor, such as an
HFC precursor, may be a reactive precursor having, a complex
structure, that significantly retains reactive precursor structure
during ion bombardment/CW plasma etching processes in the ALE of
the present invention. In some embodiments, the precursor used for
ALE, for example, an HFC precursor, minimizes ion bombardment of a
substrate layer on which the precursor is deposited.
Accordingly, in some embodiments, the precursor may be a
hydrofluorocarbon (HFC) having a reduced F/C ratio. The F/C ratio
for a precursor may be expressed by Equation I, F/C ratio=(F-H)/C
(I)
wherein F is the number of fluorine atoms, H is the number of
hydrogen atoms, and C is the number of carbon atoms in the
precursor. In some embodiments, the precursor may have a reduced
F/C ratio of about 0 (F.apprxeq.H) or equal to 0 (F=H). In other
embodiments, the precursor may have 2, 3 or 4 carbons. Exemplary
RFC precursors having a reduced F/C ratio of 0 and having 2, 3, or
4 carbons are listed in Table 1.
TABLE-US-00001 TABLE 1 Molecular formula of precursor, name, and
structure for various HFC precursors having a reduced F/C ratio of
0. Molecular Formula Name Structure C.sub.4H.sub.4F.sub.4
(E)-1,1,1,3-tetrafluoro-2-butene ##STR00001## C.sub.4H.sub.4F.sub.4
(Z)-1,1,1,3-tetrafluoro-2-butene ##STR00002## C.sub.4H.sub.4F.sub.4
2,4,4,4-tetrafluoro-1-butene ##STR00003## C.sub.3H.sub.3F.sub.3
2,3,3-trifluoropropene ##STR00004## C.sub.3H.sub.3F.sub.3
3,3,3-trifluoropropene ##STR00005## C.sub.3H.sub.3F.sub.3
1,1,3-trifluoroprop-1-ene ##STR00006## C.sub.3H.sub.3F.sub.3
1,2,3-trifluoroprop-1-ene ##STR00007## C.sub.3H.sub.3F.sub.3
1,3,3-trifluoroprop-1-ene ##STR00008## C.sub.2H.sub.3F.sub.3
1,1,1-triftuoroethane F.sub.3C--CH.sub.3 C.sub.2H.sub.3F.sub.3
1,1,2-triftuoroethane HF.sub.2C--CH.sub.2F
In some embodiments, the HFC precursor having a reduced F/C ratio
can be 3,3,3-trifluoropropene.
To realize HFC or FC precursor layer deposition on the order of an
angstrom or angstroms in thickness, a pulsed HFC or FC precursor
injection into a low power Ar plasma may be used. For example, in
some embodiments, a pulsed HFC or FC injection into a low power Ar
plasma may be used. For an unbiased substrate, precise HFC or FC
film thickness control in the about 1 .ANG., 2 .ANG., 3 .ANG., 4
.ANG., 5 .ANG., 6 .ANG., 7 .ANG., 8 .ANG., 9 .ANG., 10 .ANG. (1
nm), 1.5 nm, 2 nm, 2.5 nm, to about 3 nm range may be possible by
adjusting the total number N of HFC or FC molecules entering the
reactor of a plasma system by varying pulse duration and HFC or FC
gas flow rate appropriately. For these conditions, HFC or FC film
thickness may increase linearly with N.
Following chemical precursor deposition, an etching step may be
performed in which a small radio frequency (RF) self-bias voltage
may be applied for a predetermined length of time. For example, in
certain embodiments, a small RF self-bias voltage of -5, -10, -15,
-20, -25, -30, -35, -40, -45 and/or -50 V may be applied. As a
result, maximum ion energies of 20, 25, 30, 35, 40, 45, 50, 55, 60
and/or 65 eV, respectively, may be created. The small RF self-bias
voltage may be applied for an etching step length (ESL) of, for
example, about 15 s, about 18 s, about 20 s, about 25 s, about 30
s, or about 35 s. For the low maximum ion energies, Ar.sup.+ ion
induced physical, sputtering of unmodified surface material may be
negligible. At the end of a cycle, the process sequence may be
repeated to achieve precise control over the total etched
thickness. Any number of cycles, for example, from 1 to about 5,
10, 15, 20, 25, or 30, or more, cycles, may be performed in the ALE
procedure described herein.
To carry out ALE procedures, according to some embodiments, various
plasma systems may be used. For example, in some embodiments, an
inductively coupled plasma system may be used. In other
embodiments, the plasma system may include capacitively coupled
plasma systems, electron cyclotron resonance plasma systems,
Helicon wave plasma systems, and electron-beam generated plasma
systems. The plasma systems may also include magnetic enhancements
and controllers configured to control an amount of chemical
precursor deposition, and to control the removal of portions of the
chemical precursor layer(s), modified surface layer(s), and
controlled portion of the substrate.
The plasma system may be excited at various frequencies, both high
and low. For example, according to some embodiments, the plasma
system may be excited at, for example, 13.56 MHz. The substrate may
be biased at a radiofrequency of, for example, 3.7 MHz. The base
pressure achieved before processing may be in various ranges, such
as, for example, the 1.times.10.sup.-7 Torr range, and the
temperature of the samples may be stabilized by substrate cooling
during plasma processing at, for example, 10.degree. C.
Various materials may also be used with the plasma system. For
example, SiO.sub.2--Si--SiO.sub.2 or
SiO.sub.2--Si.sub.3N.sub.4SiO.sub.2 stacks deposited on an Si
substrate may be used. The stacks may be deposited on the Si
substrate by plasma-enhanced chemical vapor deposition (PECVD), or
any other technique as would be appreciated by one of skill in the
art, and studied by, for example, in-situ ellipsometry in real
time. In other embodiments, any of the other various substrate
materials described above may also be used.
Controlled deposition and chemical modification of the surface may
allow selective removal of a sub-nm layer of a material layer,
wherein selective removal may be represented in terms of the
material that is removed under the precursor layer, which may be
modified prior to, or is modified during, the removal process. To
establish strongly-time-dependent etch rates, a sequential approach
may be used including a thin HFC or FC precursor layer deposition
followed by a low energy Ar.sup.+ ion etch for selective removal of
volatile material.
In order to exhibit etch selectivity, in some embodiments, assisted
plasma ALE may selectively remove a first material layer/portion of
a substrate, with minimal removal/loss of a second material
layer/portion of the substrate, for example, an ALE which removes a
first material layer, for example, an SiO.sub.2 layer stacked upon
a second material layer, for example, an Si.sub.3N.sub.4 or Si
layer, in which little of or none of the second material layer is
removed/lost in the process. In some embodiments, removal or
thickness loss is less than about 5 .ANG., about 4 .ANG., about 3
.ANG., about 2 .ANG. or less than about 1 .ANG. of the second
material layer/portion.
In some embodiments, control of ALE etch selectivity may be
achieved by a combination of both increased HFC or FC layer
thickness, a shorter ESL, and a low RF bias voltage. For example,
in some embodiments, a combination of depositing an about 10 .ANG.
thick HFC or FC precursor layer, an ESL of about 18 s, and a -15 V
RIF bias voltage that generates, for example, Anions with a maximum
energy of about 30 eV, may be used for selective ALE, according to
some embodiments.
An exemplary method of ALE may be, according to some embodiments,
as follows, the method may include, I) coupling a plasma system to
deposit the controlled amount of the chemical precursor. The method
may also include, II) depositing a controlled amount of a chemical
precursor on a surface layer of a substrate to create a chemical
precursor layer and/or a modified surface layer. The method may
further include, III) establishing a gaseous environment of
sufficient purity. In certain embodiments, this can be done, in
some embodiments, by maintaining an Ar environment without
additional precursor injection. The method may also include, IV)
controlling a rate of selectively removing the portion of the
chemical precursor layer and/or a portion of the modified surface
layer, and a controlled portion of the substrate.
The method may also include, V) applying a bias potential to the
substrate at a level configured to increase ion energies. The
method may further include, VI) selectively removing a portion of
the chemical precursor layer, a portion of the modified surface
layer, and a controlled portion of the substrate in a cyclical
process. The method may also include, VII) again establishing an Ar
gaseous environment of sufficient purity. Once an Ar gaseous
environment of sufficient purity is again established, the method
may be repeated as a cyclical process beginning again from II),
until a desired overall etching depth is achieved, or a desired
amount of the substrate or a material layer on the substrate is
removed. In some embodiments, most of or a substantial portion of a
material layer etched using the ALE method is removed, i.e., the
material layer is substantially removed, such as more than 80%,
removed, more than 90% removed, and more than 95% removed, or more
than 98% removed. In some embodiments, a material layer etched
using the ALE method is completely removed, i.e., none of the
material layer remains after etching. If desired, variations on
pulse length, precursor thickness and/or etching step length (ESL)
can vary from cycle to cycle. Thus, it is not required that all
cycles be identical. However, in certain embodiments, the cycles
may be identical if desired.
Plasma systems in which the methods of the invention may take
place, as well as controllers for such systems, include, in some
embodiments: systems, including coupled plasma systems; and
controllers, such as a computer-implemented controller including a
processor and memory, such as is described in U.S. Pat. No.
9,620,382, the content of which is incorporated by reference herein
in its entirety.
Having described various aspects of the present invention, the same
will be explained, in further detail in the following examples.
Much are included herein for illustration purposes only, and which
are not intended to be limiting to the invention.
EXAMPLES
These examples below compare the ALE selectivity of SiO.sub.2 over
Si.sub.3N.sub.4 and SiO.sub.2 over Si using (1) an FC precursor,
C.sub.4F.sub.8, (2) a mixture of an FC precursor and H.sub.2,
C.sub.4F.sub.8/H.sub.2, and (3) an HFC precursor,
C.sub.3H.sub.3F.sub.3. The detailed information regarding the
chemical structure of the precursors used for this study is shown
in Table 2. The experimental tests targeted ALE selectivity of
SiO.sub.2 over Si.sub.3N.sub.4 and SiO.sub.2 over Si. The precursor
deposited film thickness per cycle is designed to be thick enough
to form a passivating layer on Si.sub.3N.sub.4 or Si, whereas the
relatively higher carbon consumption rate of SiO.sub.2 leads to
negligible film thickness on SiO.sub.2. Stacked
SiO.sub.2--Si.sub.3N.sub.4--SiO.sub.2 and SiO.sub.2--Si--SiO.sub.2
on Si substrates were used. The material thickness loss after fully
etching the top SiO.sub.2 layer before forming a passivation layer
and stopping etch was used to evaluate the ALE selectivity for
different precursors. The first part discusses the ALE behavior of
SiO.sub.2 and Si.sub.3N.sub.4 and the corresponding Si.sub.3N.sub.4
thickness loss before the formation of an etch-stopping FC
passivation layer. To study the correlation between the chemical
structure of the precursor and the surface chemistry evolution,
x-ray photoelectron spectroscopy (XPS) analysis of the materials at
the end of two ALE process steps in one cycle, i.e., the FC
deposition step and the etching step, was performed. In the second
part, SiO.sub.2 and Si etching were studied using an identical set
of precursors and Si thickness loss before etch stop was
determined.
TABLE-US-00002 TABLE 2 Molecular formula of precursor, name and
structure for the precursors used. Molecular Formula Name Structure
C.sub.4F.sub.8 octafluorocyclobutane ##STR00009##
C.sub.3H.sub.3F.sub.3 3,3,3-trifluoropropene ##STR00010##
EXPERIMENTAL
The experimental setup is briefly described here. An inductively
coupled plasma (ICP) reactor was used and excited at 13.56 MHz. The
substrate was biased at the radiofrequency (RF) of 3.7 MHz. The
plasma was confined within a 195 mm diameter anodized Al
confinement ring. A 125 mm diameter Si substrate was located 150
min below the top electrode on an electrostatic chuck. The base
pressure before processing was in the 1.times.10.sup.-7 Torr range,
and the temperature of the 25.times.25 mm.sup.2 sample was
stabilized by a cooled chuck at 10.degree. C. during plasma
processing. A load lock and vacuum transfer were used for all
experiments in order to minimize environmental impacts during the
sample exchange. The detailed configuration of the plasma system
can be found in Standaert et al., J. Vac. Sci. Technol. A 17, 741
(1999) and Engelmann et al., J. Vac. Sci. Technol. B 25, 1353
(2007). Before each experiment, an O.sub.2 plasma-based cleaning
process and Ar plasma-based conditioning process were employed to
ensure that the condition of the reaction chamber remained
consistent. Two sets of multilayer stacked samples: (1)
SiO.sub.2--Si--SiO.sub.2 and (2)
SiO.sub.2--Si.sub.3N.sub.4--SiO.sub.2 deposited on a Si substrate
enabled evaluation of etching selectivity of SiO.sub.2 over Si and
SiO.sub.2 over Si.sub.3N.sub.4. The in situ ellipsometry with a
He--Ne laser on the ICP reactor allowed for studying material
thickness changes in real time.
The ALE experiments were performed following the FC assisted ALE
procedure that is based on the use of a steady-state Ar plasma in
conjunction with periodic injections of a defined number of
precursor gases and synchronized plasma based Ar ion bombardment.
The process sequence is as follows. All plasma processes presented
in this work operate with a source power of 200 W and a processing
pressure of 10 mTorr along with a 50 sccm Ar carrier gas flow rate.
The ALE process is comprised of a deposition step and an etching
step. In the deposition step, a selected precursor was introduced
with an injection length of 2 s to deposit a 10 .ANG. thick FC film
on the substrate. This step was followed by a 12 s purge step to
pump out the residual reactive gas. In the etching step, a 15 V RF
bias voltage was employed with an etching step length (ESL) of 18
s, accelerating energetic Ar ions with a maximum kinetic energy of
30 eV to sputter the deposited FC film and reacted layer. The
process conditions of the deposition and etching steps are
optimized to obtain a good ALE selectivity of
SiO.sub.2/Si.sub.3N.sub.4 and Si by using the C.sub.4F.sub.8
precursor. Since the deposition yield may be different for each
material, the experiment was set to deposit a 10 .ANG. thick FC
film on SiO.sub.2 at the beginning cycles and keep a fixed amount
of precursor injection during the following cycles. Three sets of
precursors were explored, i.e., (1) 2 sccm C.sub.4F.sub.8, (2) 2
sccm C.sub.4F.sub.8 mixing with 3 sccm H.sub.2, and (3) 2 sccm
C.sub.3H.sub.3F.sub.3.
To establish surface chemistry modification with regard to
precursor chemistry and structure, XPS analyses have been performed
at the end of the deposition and etching steps. The samples were
transferred under vacuum to a Vacuum Generators ESCALAB MK II
surface analysis system for XPS measurements. High resolution scans
of the Si2p, C1, O1s, N1s, and F1s spectra were obtained at 20 eV
pass energy at an electron take-off angle of 20.degree. (shallow
probing depth.apprxeq.20-30 .ANG.) and 90.degree. (deep probing
depth.apprxeq.80 .ANG.) with respect to the sample surface. The
spectra were calibrated by the binding energy position of the C--C
peak to 284.5 eV and fitted using a least square fitting after
Shirley background subtraction.
RESULTS
A. SiO.sub.2 and Si.sub.3N.sub.4 ALE
An in situ ellipsometry model in the Delta-Psi space was formulated
to simulate the top SiO.sub.2 and Si.sub.3N.sub.4 etching with
corresponding accumulated FC film on the surface. This is shown in
FIG. 1(a). The solid arrow describes top SiO.sub.2 etching, and the
short, dotted line describes etching of the Si.sub.3N.sub.4
underlayer. The process condition is designed for etching top
SiO.sub.2 followed by etch stop on Si.sub.3N.sub.4 by formation of
a FC passivation layer on the surface. The dashed lines in FIG.
1(a) simulate the formation of the FC film with a given
Si.sub.3N.sub.4 loss, such as 4 and 7 nm, which is used for
evaluating the ALE selectivity.
The in situ ellipsometry data of ALE using C.sub.4F.sub.8,
C.sub.4F.sub.8, C.sub.4F.sub.8/H.sub.2, and C.sub.3H.sub.3F.sub.3
on an SiO.sub.2--Si.sub.3N.sub.4--SiO.sub.2 stack sample were
superimposed on the modeling results [FIG. 1(b)]. The ALE process
conditions were the same as described in the beginning of this
section, i.e., 10 .ANG. FC deposition thickness per cycle on
SiO.sub.2, 18 s ESL, and 15 V RF bias voltage that creates Ar ions
with a maximum energy of 30 eV. The black dots describe the
C.sub.4F.sub.8-based ALE that removes the top SiO.sub.2 layer and
gradually accumulates a FC film during the Si.sub.3N.sub.4 etching
region. Etching of Si.sub.3N.sub.4 stops once the accumulated film
is thick enough to prevent further erosion of the Si.sub.3N.sub.4
underlayer. The corresponding Si.sub.3N.sub.4 loss is used to
represent ALE selectivity using C.sub.4F.sub.8. The
C.sub.4F.sub.8/H.sub.2 ALE data show that the top SiO.sub.2 layer
is also completely removed and a FC passivation layer forms with
more Si.sub.3N.sub.4 consumption in comparison to C.sub.4F.sub.8
ALE. Other C.sub.4F.sub.8/H.sub.2 ratios including 2/1 and 2/2 sccm
were examined, and the experimental results show that any hydrogen
additive with C.sub.4F.sub.8 degrades the ALE selectivity of
SiO.sub.2 over Si.sub.3N.sub.4. On the other hand,
C.sub.3H.sub.3F.sub.3-based ALE shows a quick formation of a FC
passivation layer with negligible Si.sub.3N.sub.4 thickness loss
after top SiO.sub.2 etching. Each of these three precursors enabled
continuous ALE of SiO.sub.2 and accumulated FC passivation on the
Si.sub.3N.sub.4 surface that prevents further Si.sub.3N.sub.4
loss.
The SiO.sub.2 and Si.sub.3N.sub.4 layer thickness at the end of
each cycle using C.sub.4F.sub.8, C.sub.4F.sub.8/H.sub.2 and
C.sub.3H.sub.3F.sub.3 ALE is shown in FIG. 2(a). The ellipsometry
model shown in FIG. 1(a) was applied to fit the experimental in
situ ellipsometry data and measure the SiO.sub.2 and
Si.sub.3N.sub.4 thickness change. For SiO.sub.2 etching,
C.sub.4F.sub.8 and C.sub.4F.sub.8/H.sub.2-based ALE gave higher
SiO.sub.2 thickness etched per cycle (EPC) than
C.sub.3H.sub.3F.sub.3. Si.sub.3N.sub.4 EPC showed a strong
dependence on the gaseous precursor used for ALE. C.sub.4F.sub.8
mixing with H.sub.2 exhibited the highest Si.sub.3N.sub.4EPC,
followed by C.sub.4F.sub.8 and C.sub.3H.sub.3F.sub.3. The use of a
C.sub.3H.sub.3F.sub.3-based ALE process shows minimum
Si.sub.3N.sub.4 loss by rapid formation of a FC passivation layer,
whereas the C.sub.4F.sub.8/H.sub.2 mixture using identical process
condition increases Si.sub.3N.sub.4 material loss. A comparison of
the amounts of net Si.sub.3N.sub.4 loss for C.sub.4F.sub.8,
C.sub.4F.sub.8/H.sub.2, and C.sub.3H.sub.3F.sub.3-based ALE at the
end of the 28th cycle, when C.sub.3H.sub.3F.sub.3-based ALE
obtained a full etch stop, is shown in FIG. 2(b). C.sub.4F.sub.8,
C.sub.4F.sub.8/H.sub.2C.sub.3H.sub.3F.sub.3-based ALE gave 37, 70
and 4 .ANG. Si.sub.3N.sub.4 loss, respectively.
To evaluate surface chemical aspects of these processes, XPS
spectra for Si.sub.3N.sub.4 ALE using C.sub.4F.sub.8,
C.sub.4F.sub.8/H.sub.2, and C.sub.3H.sub.3F.sub.3 at the end of the
deposition and etching steps were obtained. Scans were performed at
the beginning of Si.sub.3N.sub.4 etching after the removal of the
top SiO.sub.2 layer, for example, the 8th cycle for C.sub.4F.sub.8,
the 9th cycle for C.sub.4F.sub.8/H.sub.2 and the 12th cycle for
C.sub.3H.sub.3F.sub.3 its FIG. 2(a). The data shown in FIG. 3
summarize the Si2p, C1s, and F1s spectra for the set of precursors.
XPS intensity difference spectra corresponding to XPS data after
etching and deposition steps, respectively, are shown. The Si2p
spectra were fitted using fluorinated silicon nitride (Si--NT) and
fluorinated silicon oxide (Si--OF) at 102.4 and 104.2 eV,
respectively. The C1s spectra were fitted using C--C/C--H.
C--CF.sub.x, C--F/C--O, C--F.sub.2, and C--F.sub.3 at 284.5, 286.2,
287.9, 290.2, and 292.6 eV, respectively. The F1s spectra were
decomposed into F--SiON and F--C at 686.8 and 687.8 eV,
respectively. XPS intensity differences between after etching and
deposition data for C.sub.4F.sub.8, C.sub.4F.sub.8/H.sub.2 and
C.sub.3H.sub.3F.sub.3 were evaluated to clarify the extent of
surface reaction by Ar ion bombardment. The spectra were normalized
by the total areal intensity of all elements corrected with the
relative sensitivity factors on the surface, including Si2p, C1s,
N1s, O1s, and F1s.
A comparison of the F/C ratios calculated from the C1s and F1s/C1s
spectra at a take-off angle of 20.degree. with C.sub.4F.sub.8,
C.sub.4F.sub.8/H.sub.2, and C.sub.3H.sub.3F.sub.3-based ALE at the
end of deposition and etching steps, respectively, is shown in FIG.
4(a). The F/C ratios from the C1s spectra measures the fluorine
content in the deposited FC layer by calculating the area
percentage of weighted C--F, CF.sub.2 and CF.sub.3 peaks in the
spectra. The ratio of F1s/C1s spectra measures the fluorine content
in both FC layer and underlying Si.sub.3N.sub.4. By observing the
difference between the F/C ratios from the C1s spectra and F1s/C1s
spectra, the fluorine reacted with the underlying Si.sub.3N.sub.4
was evaluated. FIG. 4(b) measures the thickness of the HFC film
remained on the Si.sub.3N.sub.4 surface after the Ar ion
bombardment by angle-resolved XPS (AR-XPS) using the
Si2p signal attenuation obtained from 20.degree. and 90.degree.
take-off angles.
The C1s spectra after FC deposition in FIG. 3(b) show that
C.sub.4F.sub.8 leads to strong C F/C--O, C--F.sub.2, and CF.sub.3
peak intensity, whereas mixing H.sub.2 with C.sub.4F.sub.8 in FIG.
3(e) produce surfaces that exhibit relatively low CF.sub.x peak
intensity. Importantly, C.sub.3H.sub.3F.sub.3 overall minimizes the
CF.sub.x peak intensities, i.e., fluorine content, for the
deposited HFC film shown in FIG. 3(h). The comparison of the F/C
ratios from the C1s spectra after deposition step also shows that
C.sub.4F.sub.8-based FC film has the highest fluorine content,
followed by C.sub.4F.sub.8/H.sub.2 and C.sub.3H.sub.3F.sub.3 [FIG.
4(a)]. Since the ratio of F1s/C1s spectra is almost identical to
the F/C ratios from the C1s spectra, most of the fluorine measured
is from the deposited film and has not reacted with the underlying
Si.sub.3N.sub.4. The C1s spectra and F/C ratios confirm that the
precursor chemistry used for ALE controls the fluorine content in
the deposited film. The results suggest that gaseous precursors
with same degree of unsaturation or high F/C ratio can lead to
fluorine-rich deposited FC films.
FC film defluorination and reacted material removal take place for
C.sub.4F.sub.8, C.sub.4F.sub.8/H.sub.2, and
C.sub.3H.sub.3F.sub.3-based ALE during the etch step. The C1s
spectra show the intensity reduction of the C--Fx peaks in FIGS.
3(b), 3(e), and 3(h). The intensity of the C--C/C--H peak increase
is related with the FC film defluorination during etching, since
the release of fluorine from the film contributes to the formation
of a carbon rich layer. C.sub.3H.sub.3F.sub.3 showed a stronger
C--C/C--H peak intensity after the etching step, implying that a
carbon rich passivating layer remains on the surface in comparison
to C.sub.4F.sub.8 and C.sub.4F.sub.8/H.sub.2. The F/C ratio from
the C1s spectra after etching step [FIG. 4(a)] also shows the
reduction of the fluorine content in the deposited HFC film. At the
same time, the etching step also reduces the deposited FC film
thickness, so the underlying Si.sub.3N.sub.4 will be exposed more
to the fluorine etchant, introducing fluorination and etching of
the reacted Si.sub.3N.sub.4. The Si2p and F1s spectra after
C.sub.4F.sub.8-based ALE [FIGS. 3(a) and 3(c)] exhibit increased
Si--NF and Si--OF peak intensities after the etching step. The F/C
ratios from the F1s/C1s spectra is relatively higher than the
ratios from the C Is spectra after etching step [FIG. 4(a)]
supports that the fluorine etchant reacts with the underlying
Si.sub.3N.sub.4 and results in Si.sub.3N.sub.4 fluorination. With
the confirmation from the AR-XPS FC thickness measurements [FIG.
4(b)], it was concluded that etching of the FC film and
fluorination of the Si.sub.3N.sub.4 underneath takes place.
Comparison of C.sub.4F.sub.8, C.sub.4F.sub.8/H.sub.2 shows less
remained HFC film and higher fluorination of the underlying
Si.sub.3N.sub.4. Surfaces after C.sub.3H.sub.3F.sub.3-based ALE
show least etching of the FC layer and negligible fluorination of
the underlying Si.sub.3N.sub.4.
The XPS difference spectra after FC film deposition and etching for
C.sub.4F.sub.8, C.sub.4F.sub.8/H.sub.2 and C.sub.3H.sub.3F.sub.3
were used to clarify the extent of surface reactions and are shown
in FIGS. 3(j)-3(l). The spectra were normalized to the sum of the
Si2p, C1s, N1s, O1s, and F1s areal intensities corrected with the
relative sensitivity factors. The Cis difference spectra in FIG.
3(k) confirms that C.sub.3H.sub.3F.sub.3-based ALE deposits contain
the minimum amount of fluorine in the HFC film, whereas the use of
H.sub.2 reduces both the F/C ratio of the deposited film and
overall amount of the FC layer as compared to C.sub.4F.sub.8. The
fact that H.sub.2 enhances the removal of the remaining FC film for
the Si.sub.3N.sub.4 surface and causes less FC film coverage can be
seen from the lowered intensity of the C--C/C--H peak.
The fluorination level of the underlying Si.sub.3N.sub.4 due to the
fluorine etchant released from the deposited FC film as a result of
Ar ion bombardment is mirrored by the areas of the Si--NF, Si--OF,
and F--SiON peaks in the Si2p and F1s intensity difference spectra
shown in FIGS. 3(j) and 3(l). In the F1s intensity difference
spectra, C.sub.4F.sub.8 and C.sub.4F.sub.8/H.sub.2 show strong
fluorination on the underlying Si.sub.3N.sub.4, which is the
reacted layer that will be etched in the etching step, whereas
C.sub.3H.sub.3F.sub.3 shows little SiON--F peak formation. The
formation of the F--SiON peak in the F is spectra may be correlated
with the Si.sub.3N.sub.4 loss before formation of a FC passivation
layer and etch stop. Since Ar ion bombardment during the etching
step causes removal of fluorinated products, the Si.sub.3N.sub.4
loss is positively correlated to the F--SiON peak area of the
material. The XPS difference spectra qualitatively agree with the
SiO.sub.2 and Si.sub.3N.sub.4 thickness changes measured by
ellipsometry. For C.sub.4F.sub.8 and C.sub.4F.sub.8/H.sub.2-based
ALE, more Si.sub.3N.sub.4 loss is required before a sufficiently
thick FC passivating layer is established. The classical concept of
gas mixing by combining C.sub.4F.sub.8 and H.sub.2 does not reduce
the material loss for FC-assisted ALE because of the strong loss of
the surface passivation layer as a result of hydrogen induced
etching of the FC film.
B. SiO.sub.2 and Si ALE
Selective ALE of SiO.sub.2 over a Si underlayer was examined using
the identical set of precursors just discussed for
SiO.sub.2--Si.sub.3N.sub.4--SiO.sub.2 stacks. Results for
SiO.sub.2--Si--SiO.sub.2 stacks are shown in FIG. 5(a). For the FC
assisted ALE based on C.sub.4F.sub.8, the Si thickness loss before
establishing a sufficiently thick FC film to stop Si removal is
about 9 .ANG.. When C.sub.4F.sub.8 was mixed with Hz, a higher
thickness EPC of SiO.sub.2 and Si is observed, and the Si thickness
loss increases to 40 .ANG. at the 30th cycle. For
C.sub.3H.sub.3F.sub.3-based ALE, a comparable SiO.sub.2 EPC to that
when using C.sub.4F.sub.8 is observed. However, in this case, a
reduced Si EPC is seen for C.sub.3H.sub.3F.sub.3 once the process
starts to etch Si. No more Si loss or Si EPC reduced to a
negligible amount of less than 1 .ANG. is observed after the 19th
cycle. This shows that for a C.sub.3H.sub.3F.sub.3 ALE precursor an
HFC passivation layer can rapidly build up and prevent Si loss.
For a selective SiO.sub.2 ALE process, the Si thickness loss should
be minimal. The precursor dependence of Si thickness loss before Si
etching stop is shown in FIG. 5(b). C.sub.3H.sub.3F.sub.3 has the
minimum Si thickness loss (2 .ANG.) as compared to the other
precursors examined (C.sub.4F.sub.8 10 .ANG. and
C.sub.4F.sub.8/H.sub.2 40 .ANG.). This result reveals that high ALE
selectivity can be achieved by optimizing the chemical structure
and composition of the reactive film using different precursors.
These results also indicate that precursor mixing used in
continuous plasma etching for improving SiO.sub.2 over Si ALE
selectivity is ineffective for FC-assisted ALE since the times
required for suppressing substrate etching are impractically
long.
The foregoing is illustrative of the present invention and is not
to be construed as limiting thereof. The invention is defined by
the following claims, with equivalents of the claims to be included
therein.
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